1. Field of the Invention
The present invention generally relates to a color conversion technology of CMYK data, and more particularly, to a color converting device, a color converting method, and a recording medium storing color printer software operated in a workstation, for converting CMYK print data (a color image signal) generated in an image forming device, such as a color copying machine, a color printer, or a color facsimile, into color data (a color signal) in a different format suitable for displaying on a color display or printing from another color printer.
2. Description of the Related Art
There are a lot of conventional technologies regarding a color conversion of CMYK data.
For example, a memory map interpolative calculation method using a four-dimensional lookup table is one of general color converting methods for converting color data (CMYK image data) consisting of four color components such as CMYK into RGB image data. (e.g., Japanese Patent No. 2,903,808 and Japanese Laid-Open Patent Application No. 57-208765)
The above-mentioned memory map interpolative calculation method realizes a highly precise color conversion throughout a color space. The method comprises the steps of dividing a four-dimensional color space formed by four color signals into a plurality of five-vertex solids, selecting a five-vertex solid including an input color by using higher-order bits of input data (an input signal), and reading an interpolative calculation intensity corresponding to the selected five-vertex solid from a four-dimensional lookup table so as to perform a linear interpolative calculation.
Other means for converting CMYK image data into RGB image data include a device described in Japanese Laid-Open Patent Application No. 9-284579. This device performs a four-dimensional interpolative calculation by linearly interpolating results of a plurality of three-dimensional interpolative calculations. For example, the four-dimensional interpolative calculation can be performed to input CMYK signals by performing a plurality of three-dimensional interpolative calculations of a C-M-Y space, selecting two of results of the three-dimensional interpolative calculations according to higher-order bits of a K signal, and linearly interpolating the selected two results by lower-order bits of the K signal.
However, the above-mentioned memory map interpolative calculation method, which divides a cubic unit into 24 five-vertex solids, requires judging processes in 24 patterns using lower-order bits of the input signal. Although these judging processes can be processed in parallel when performed by means of hardware, these judging processes claim a long time to successively compare the lower-order bits with each other to find a larger bit in each (magnitude relation) comparison when performed by means of software (in a CPU).
As to the above-mentioned device described in Japanese Laid-Open Patent Application No. 9-284579, since the device performs a plurality of the three-dimensional interpolative calculations in parallel, the three-dimensional interpolative calculations claim quite a long time when processed by a CPU. For instance, performing a four-dimensional interpolative calculation in which each of axes C, M, Y and K is divided into 8 entails a total of nine three-dimensional interpolative calculations corresponding to K signals in nine patterns.
Also, there is a method for printing CMYK print data by another color printer. This method comprises the steps of converting CMYK data into a device-independent Lab signal, correcting the Lab signal so as to faithfully reproduce a ground color, performing a contrast control for enhancing a visibility of a highlight when not reproducing the ground color, and thereafter converting the Lab signal into CMYK data used for another color printer. (Japanese Laid-Open Patent Application No. 8-212324)
It is well known that a hardcopy image printed on such a printer as a color printer and a softcopy image displayed on such a display as a CRT display are perceived as differently looking colors because of utterly different color reproducing methods applied to the images, though the images are based on same image data. Thereupon, there is a method for making the hardcopy image and the softcopy image perceived as identically looking colors. This method comprises the steps of equalizing a dynamic range of input color data and a dynamic range of output color data, creating a color conversion profile by using a chromatic adaptation model so that the hardcopy image and the softcopy image become perceptually identical, and performing a color conversion by using the color conversion profile. (Japanese Laid-Open Patent Application No. 11-112819)
Some of image forming devices such as color printers and color copying machines can generate CMYK data in the devices themselves and store the CMYK data in an inner storage device such as a hard disk so as to use the CMYK data repeatedly. This stored CMYK data can be converted into RGB data more reusable in a computer so as to be displayed as a softcopy image on a display connected to the computer. Furthermore, this RGB data displayed on the display can be processed as by adding characters or symbols thereto, and this processed digital image data can be reproduced again as a hardcopy.
However, simply converting the stored CMYK data into the RGB data and transmitting the RGB data to the computer so as to display the RGB data on the display or print the RGB data on another color printer may not possibly result in a high-quality image. Especially the reproduction quality of a black character is noticeably deteriorated.
This is because the CMYK data generated in the image forming device has a characteristic optimized for a hardcopy. For example, when a color copying machine prints a black character by overlapping four colors of CMYK, displacement of each color reduces the sharpness and legibility of the black character. Thereupon, the color copying machine often performs an image area separation process which extracts edge parts of a black character so as to print the edge parts monochromatically with the color of K, and separates the color of other pictorial parts into the four colors of CMYK so as to use inks of all the four colors. Likewise, a color printer performs an object-suited chromatic process which judges a draw code in a printer driver so as to print a black character monochromatically with the color of K, and separate the color of other parts into the four colors. However, simply converting such CMYK data including a black character part and other parts undergoing different color reproduction methods into RGB data, etc. cannot gain a high-quality black character reproduction in displaying on a display or printing from another color printer.
Additionally, the above-mentioned technology disclosed in Japanese Laid-Open Patent Application No. 8-212324 basically enables a faithful color reproduction by converting CMYK data into colorimetrically equal CMYK data, because the technology is aimed at a conversion between hardcopy output signals. However, in a case where the CMYK data is converted into RGB data used for displaying and then is output to a color printer via a printer driver, the printer driver often performs a color correction equalizing a dynamic range of a monitor to a dynamic range of a color printer output. Accordingly, simply equalizing reference whites of CMYK data and RGB data results in an insufficient density of black in the printer output.
On the other hand, since the above-mentioned technology disclosed in Japanese Laid-Open Patent Application No. 11-112819 equalizes a dynamic range of input color data and a dynamic range of output color data, this technology can convert the input color data into the output color data suitable for a screen display, and also can prevent the above-mentioned decrease in black density when outputting the color data to another color printer via a printer driver. However, since this conventional technology is aimed at input color data read from a hard copy by a scanner or input by a digital camera, the technology cannot be applied to CMYK data subjected to the above-described image area separation process or the above-mentioned object-suited chromatic process because of problems such as that the technology cannot determine a black point (a darkest point) of the CMYK data.